Chromate conversion coating



United States Patent 3,282,744 CHROMATE CONVERSION COATING Fred Corwin Goldsmith, Painesville, Ohio, assignor to The Lubrizol Corporation, Wickliffe, Ohio, a corporation of Ohio No Drawing. Filed May 8, 1963, Ser. No. 278,990 12 Claims. (Cl. 1486.2)

The present invention relates to a composition adapted for the corrosion-proofing of metal articles. In a more particular sense, it relates to a convenient and economical method for inhibiting the corrosion of metal articles, especially ferrous metal articles and galvanized and/or phosphated ferrous metal articles.

It is known to treat metal articles, especially ferrous metal and galvanized ferrous metal articles, with aqueous phosphating solutions, aqueous chromic acid solutions, aqueous metal nitrite solutions, and the like to improve the corrosion resistance of the metal article and extend the useful life of a subsequently applied siccative top-coat such as a top-coat of paint, lacquer, enamel, varnish, synthetic resin, etc. One major shortcoming of known methods of corrosion-proofing has been noted, for example, in the automotive and household appliance industries, where ferrous metal, galvanized ferrous metal, or phosphated ferrous metal parts are invariably provided with a siccative top-coat of lacquer or enamel. It has been observed that when such a top-coat is scratched or scored during, for example, handling, forming, or assembling operations, the metal substrate becomes a focal point for corrosion and for a phenomenon known as undercutting. Undercutting, or the loosening of the top-coat in areas adjacent to a scratch or score, causes a progressive flaking of the top-coat from the affected area. In severe cases, the undercutting may extend an inch or more from each side of the scratch or score, causing a loosening and subsequent flaking of the top-coat from a substantial portion, if not. all, of the metal article.

Another shortcoming, particularly troublesome to the household washing machine industry, is the tendency of top-coated metal parts which come in contact with aqueous cleaning solutions such as aqueous solutions of common soap or synthetic detergent to develop blisters in the top-coat due to the corrosion of the metal substrate. Such blisters grow in size as the corrosion pro gresses and, in some instances, flake from the rest of the top-coat and thereby expose portions of the bare metal substrate.

It is an object, therefore, of the present invention to provide an improved corrosion-proofing composition for metal articles.

Another object is to provide a method for reducing the incidence of undercutting and'blistering in top-coated metal articles.

Still another object is to provide metal articles which exhibit improved resistance to the ravages of corrosion.

These and other objects of the invention are realized by the provision of a composition adapted for the corrosion-proofing of metal articles which consists essentially of a dilute aqueous solution of (A) partially reduced chromic acid in which the ratio of hexavalent chromium to trivalent chromium is within the range from about 0.5 to about 5, preferably from about 0.8 to about 3, and

(B) calcium dichromate;

in which solution from about 20 to about 80 percent and preferably from about 30 to about 70 percent of the chromium content thereof is contributed by component A, the remainder of the chromium content being contributed by component B.

Such dilute aqueous solution will generally contain Wee from about 0.01 to about 0.3 percent and more often from about 0.03 to about 0.1 percent of chromium present as components A and B. i

In a preferred embodiment, the invention is directed to a composition adapted for the corrosion-proofing of metal articles which consists essentially of a dilute aqueous solution of (A) partially reduced chromic acid prepared by reducing chromic acid with methanol, in which partially reduced chromic acid the ratio of hexavalent chromium to trivalent chromium is about 1, and

(B) calcium dichromate;

which solution contains from about 0.03 to about 0.1 percent of chromium and in whichsolution about half of said chromium content is contributed by component A. It has been found that components A and 'B act in a synergistic manner; that is, a combination of these components is much more effective in reducing corrosion than is either component when used alone. When used to treat galvanized steel prior to the application thereto of a topcoat of paint or enamel, the corrosion-proofing composition of the present invention completely eliminates undercutting. COMPONENT A As indicated previously, this component is partially reduced chromic acid in which the ratio of hexavalent chromium to trivalent chromium is within the range from about 0.5 to about 5, preferably from about 0.8 to about 3. It is known, as exemplified by US. Patents Nos. 2,768,104 and 3,063,877, to reduce chromic acid partially by means of oxidizable compounds such as formaldehyde, ethanol, glycol and other polyalcohols, phenol, hydroquinone, potassium iodide, etc., and then to use a dilute solution of such partially reduced chromic acid as a corrosion-proofing composition. The preparation of component A does not constitute the invention claimed herein and such preparation may be effected by known methods set forth in the cited patents or elsewhere in the literature. Example I of US. Patent 3,063,877, for example, describes the preparation of a partially reduced chromic acid (ratio of hexavalent chromium to trivalent chromium: 1.27) by the reaction below 180 F. of a solution of chromic acid (prepared by dissolving 170 pounds of CrO in 40 gallons of water) with a solution of 48 pounds of 36.6 percent aqueous formaldehyde in 21 gallons of water. From the standpoint of convenience and economy, however, it is generally preferred to use methanol as the reducing agent for chromic acid in preparing component A. The following examples are presented to illustrate additional specific modes of preparing partially reduced chromic acid useful for the purposes of the pres ent invention. These examples are presented for purposes of illustration only and are not to be construed as limiting the scope of the invention, except as the appended claims may require. Unless otherwise indicated, all parts and percentages are by weight.

Example 1 432 parts of water is introduced into a reaction vessel and 550 parts of CrO is added thereto over a period of 20 minutes at 27-32 C. The whole is heated to 93 C. over a period of 1.5 hours and a mixture of 29 parts of methanol with 29 parts of Water is added thereto be neath the surface of the chromic acid solution over a period of 4 hours at 99 C. A very exothermic reaction occurs and the addition must be made carefully to avoid a too vigorous reaction. Approximately 40 parts of carbon dioxide is formed as a consequence of the complete oxidation of the methanol. After all of the methanol has been added, the whole is stirred for 1 hour at 96 C. The product, a concentrated aqueous solution of I 3 partially reduced chromic acid, shows the following analyses.

Percent chromium 28.45 Percenthexavalent chromium 19.55 Percent trivalent chromium 8.90 Ratio of hexavalent chromium to trivalent chromium 2.2

Example 2 I To 1100 grams of CrO dissolved in 2500 grams of water and heated to 90-100 CL, there is added dropwise a mixture of 128 grams of methanol and 128 grams of water. The highly exothermic reaction maintains the temperature at 90-400 C. After all of the aqueous methanol has been added (approximately 8 to 10 hours required), the whole is stirred for an additional hour to insure complete reaction. The material in the reaction vessel is then diluted with sufficient water to yield 3 liters of solution. The resulting product, a concentrated aqueous solution of partially reduced chromic acid, shows the following analyses.

Percent chromium 14.75 Ratio of hexavalent chromium to trivalent chromium 1.15

Example 3 An experiment is carried out in the same manner set forth in Example 2, except that a mixture of 89.6 grams of methanol and 89.6 grams of water is employed. The

resulting product, a concentrated aqueous solution of partially reduced chromic acid, shows the following analyses.

Percent chromium 14.9 Ratio of hexavalent chromium to trivalent chromium 1.68

Example 4 resulting product, a concentrated aqueous solution of' partially reduced chromic acid, shows the following analyses.

Percent chromium 15.45 Ratio of hexavalent chromium to trivalent chromium COMPONENT B As indicated earlier, this component is calcium dichromate. Although anhydrous calcium dichromate (CaCr O may be employed as component B, it is generally more convenient to use a concentrated aqueous solution thereof, because such solution dissolves immediately in water upon mixing- For example, the reaction of a slurry of 1.72 parts. of calcium hydroxide in 6.5 parts of water with 5.1 parts of CrO for about 8 hours at 110120 F. yields an aqueous concentrate containing 45 percent of calcium dichromate.

As noted hereinabove, the corrosion-proofing composition of this invention consists essentially of a dilute aqueous solution of components A and B. Such dilute solutions, will ordinarily contain from about 0.01 to about 0.3 percent of chromium. When less than about 0.01 percent of chromium is present, the corrosion-proofing characteristics of the solution fall off quite rapidly. More than 0.3 percent of chromium can be present; however, the use of such larger content of chromium is uneconomical and, in most instances, is not attended by any substantial increase in the corrosion-proofing characteristics of the solution. Preferably, the chromium content of the solution. will'be within the range from about 0.03 to about 0.1 percent. Each of components A and B contributes to the chromium content of the solution. In order for the objects of the invention to be achieved, it

is important that from about 20 to about percent and preferably from about 30 to about 70 percent of the chromium content of the solution be contributed by component A, with the remainder being contributed by component B. When components A and B are present in such amounts, they act in a synergistic manner to provide optimum corrosion-proofing characteristics. Itis likewise important that the ratio of hexavalent chromium to trivalent chromium in component A be within the range from about 0.5 to about 5, preferably from about 0.8 to about 3.

A number of specific examples of aqueous corrosionproofing compositions of this invention are given in Table I. They are prepared by simply dissolving the indicated components in water.

TABLE I.DILUTE AQUEOUS CORROSION-PROOFING COMPOSITIONS OF THE INVENTION Component A Component B (calcium dichromate), Example Present in an present in an No. amount to Supamount to sup- Identity ply the indiply the indicated percent cated percent of chromium of chromium Product of Example 2.-. 0. 025 0. 025 Product of Example 3. 0. 025 0. 025 Product of Example 4 0. 025 O. 025 Product of Example 2... 0. 05 0. 025 Product of Example 3 0. 075 0. 025 Product of Example 4 0. 03 0. 09 11 Product of Example 2 0. 09 0.18 12 Product of Example 3.-. 0. 08 0. 08

The treatment of the metal article with the corrosionproofing composition of this invention may be accomplished in any one of several ways. The composition may be contacted with or applied to the article by spraying, brushing, dipping, flow-coating, roller-coating, etc., techniques, and the temperature of the composition may vary within wide limits, e.g., from room temperature or below to about 212 F. Higher temperatures such as 250 F., 300 F., or more may also be employed by the use of superatmospheric pressures. In general, best results are obtained when the temperature of the composition is within the range from about 60 F. to about F. The time of contact between the metal article and the composition of this invention is not critical and may vary from as little as 5 or 10 seconds to as much as 10 minutes or more. In most instances a contact time between about 15 seconds to about 1 or 2 minutes suffices. For best results, the metal article should be thoroughly clean before it is contacted with the corrosion-proofing composition. Cleaning, if required, may be accomplished by known physical and/or chemical means which remove grease, dirt, oxides, and other surface contaminants.

After the metal article has been treated with the composition of this invention, it is dried. This can be done by allowing it to drain and dry at ambient temperature, by subjecting it to a current of hot air, by passing it through a heated zone, etc. The treated and dried metal article is then generally provided with a siccative topcoat such as a top-coat of paint, enamel, varnish, lacquer, synthetic resin, etc., to provide further protection and/or decorative effects. Such top-coat may be applied by conventional means such as spraying, brushing, dipping, roller-coating, etc., and then dried either by exposure to air or by means of a baking technique, depending on the nature of the siccative top-coat material.

The following experiments were carried out to determine the corrosion-proofing characteristics of compositions of this invention. They are presented for purposes of illustration only and are not intended to limit the scope of the invention, except as the appended claims may require. Unless otherwise specified, all parts and percentages are by weight.

Example 13 Three clean, 4-inch by 1'2-inch panels of satin finish, galvanized ZO-gauge SAE 1020 steel were spray-phosphated for one minute at 150 F. with an aqueous phosphating solution containing 0.091% of calcium ion, 0.029% of zinc ion, 0.71% of phosphate ion, 0.034% of nickel ion, and 0.13% of nitrate ion, and having a total acidity of 12.7 points (points total acidity represents the number of milliliters of 0.1 normal sodium hydroxide solution required to neutralize a milliliter sample of .the phosphating solution in the presence of phenolphthalein indicator). The phosphated panels were sprayrinsed with water for seconds at room temperature and then spray rinsed for 15 seconds at room temperature, respectively, with a corrosion-proofing composition of this invention (Example 6; contains 0.05% of chromium) and two control solutions prepared by separately dissolving component A and component B of Example 6 in sufficient water to yield solutions containing 0.05% of chromium. For c-onvenie-nt identification, the panels were labeled, respectively, as panels E, F, .and G. After the panels had been treated, they were dried in a current of ho-t (300 F.) -air for 10 seconds and dipped in a gray, epoxy resin-based bake enamel. The top-coated panels were then baked in an oven for 10 minutes at 400 F. and subjected to the Tide Corrosion test. In this test, panels to be evaluated are immersed for 96 hours at 160 F. in an aqueous detergent solution consisting of water plus 1 percent of a synthetic detergent composition consisting of 53 parts of sodium pyrophosphate decahydra-te, .19 parts of sodium sulfate, parts of sodium alkylbe-nzene sulfonate, 7 parts of sodium metasilicate, and 1 part of sodium carbonate. Upon removal from the test solution, the panels are rated for blistering of the top-coat by means of ASTM Procedure D 714-56. The rating provides for the designation of the size of any blisters present on a scale of 0 to 10, wherein 0 represents large blisters and 10 represents no blisters. It also provides for the firequency of any blisters present as follows: dense=D, medium dense=MD, medium=M, and few=F.

The test results obtained on the several panels of this example are set forth in Tab-1e II.

TABLE II.TIDE CORROSION TEST Top-Coated Panel Blister Blister Size Frequency E (of the invention) 10 none F (control) 8 MD G (control) 9 F Three clean, galvanized steel panels of the type described in Example 13 were spray-rinsed with Water for 15 seconds at room temperature and then spray-rinsed for 15 seconds at room temperature, respectively, with a corrosion-proofing composition of this invention (Example 5; contains 0.05 of chromium) and two control solutions prepared by separately dissolving component A and component B of Example 5 in sufiicient water to yield solutions containing 0.05% of chromium. For convenient identification, the panels were labeled, respectively, as panels H, I, and J. The panels were dried, top-coated, baked, and tested in the same manner settorth in Example 13.

The test results obtained on the several panels are shown in Table III.

TABLE IIL-TIDE CORROSION TEST Top-Coated Panel Blister Blister Size Frequency H (of the invention)" 9 F I (control) 8 MD I (control) 5 MD Three clean, 4-inch by 12-inch panels of spangle finish, galvanized 20-gauge SAE 1020 steel were spray-rinsed with water for 15 seconds at room temperature and then spray-rinsed for 15 seconds-at room temperature, respectively, with a corrosion-proofing composition of this invention (Example 5; contains 0.05 of chromium) and two control solutions prepared by separately dissolving component A and component B of Example 5 in sutficient water to yield solutions containing 0.05 of chromium. For convenient identification, the panels were labeled, respectively, as panels K, L, and M. The treated panels were :dried in a current of hot (300 F.) air for 10 secends and then sprayed with a white, alkyl resin-based bake enamel. The top-coated panels were baked in an oven for 20 minutes at 320 F., scribed with a sharp steel instrument to yield a vertical score extending one inch from the top of the panel to one inch from the bottom thereof, and subjected to the Salt Fog Corrosion test. The apparatus used for this test is described in ASTM Procedure B 117-57T. It consists of a chamber in which a mist or fog of 5 percent aqueous sodium chloride is maintained in cont-act with the test panels for a predetermined time (in the present case, 120 hours) at i2 F. Upon removal from the test chamber, the top-coated panels are inspected and rated for undercutting. The

under-cut rating is the average loss of enamel from each side of the score expressed as a value which represents the number'of thirty-seconds of an inch of such loss.

The results obtained in this test are shown in Table IV.

TABLE IV.-SALT FOG CORROSION TEST, HOURS Top-coated panel: Under'cut rating K (of the invention) 0.0 L (control) 6.0 M (control) 7.0

It will be noted that the panel which had been treated with a corrosion-proofing composition of this invention showed no undercutting, whereas panels which had been treated with separate solutions of components A and B of said composition showed substantial loss of enamel due to undercutting.

Example 16 Top-coated panel: Undercut rating N (of the invention) 0.0 0 (control) 9.0 l (control) 7.0

Again, the panel which had been treated in accordance with the present invention gave superior results.

7 Example 17 TABLE VL-SALT FOG CORROSION TEST, 120 HOURSv Undercut rating Top-coated panel:

Q (of the invention) 0.0 R (control) 2.5 S (control) 7.0

Superior results were again obtained with the panel which hadbeen treated in accordance with the present invention.

Example 18 Example 17 was repeated using plain steel panels instead of galvanized steel panels. The panel treated with the composition of this invention was labeled panel T; the control panels were U and. V.

The test results are set forth in Table VH.

TABLE VII.-SALT FOG CORROSION TEST, 120 HOURS Top-coated panel: Undercut rating T (of the invention) 5.0 U (control) 10.0 Vtcontrol) 14.0

The experiment shows that the corrosion-proofing composition of this invention is effective in protecting plain steel as well as galvanized or phosphated steel- In addition to their utility as corrosion-proofing compositions forappllication to, the surfaces of ferrous metal articles, galvanized ferrous metal articles, and phosphated metal articles, the corrosion-proofing compositions-of this invention are also useful in protecting non-ferrous metals and alloysthereof such, as aluminum, magnesium, cadmium, copper, brass, bronze, white metal, etc, against corrosion. They are also useful in protecting plated metal surfaces such as copper-plated, nickel-plated, and cadmium-plated ferrous metal articles against corrosion.

What is claimed is:

1. A composition adapted for the corrosion-proofing of metal articles which consists essentially of a dilute aqueous solution of (A) partially reduced chromic acid in which theratio 8 of hexavalent chromium to trivalent chorniurn is within the range from about 0.5 to about 5, and

( B) calcium dichr-omate In which solution from about 20 to about 80 percent of the chromium content thereof is contributed by component A.

2 A composition in accordance with claim 1 which contains from about 0.01 to about 0.3 percent of chrornium.

3. A composition in accordance with claim 1 wherein the ratio of hexavalent chromium to trivalent chromium in component A is within the range from about 0.8 to about 3.

4. A composition in accordance with claim 1 wherein from about 30'to about percent of the chromium content thereof is contributed by component A.

5. A composition in accordance with claim 1 wherein component A .is prepared by partially reducing chromic acid with methanol.

6. A composition adapted for the corrosion-proofing of metalarticles which consists essentially of a dilute aqueous solution of (A) partially reduced chromic acid prepared by reducing chromic acid with methanol, in which partially reduced chromic acid the ratio of hexavalent chromium to trivalent chromium is about 1, and

(B) calcium dichromate; which solution contains from about 0.3 to about 0.1 percent of chromium and in which solution about half of said chromium content is contributed by component A.

7. A method of corrosion-proofinga metal article which comprises contacting said metal article with the composition of claim 1.

8. A method in accordance with claim 7 wherein the metal article is a ferrous metal article.

9. A method in accordance with claim 7 wherein the metal .article'is a galvanized ferrous metal article.

10. A method in accordance with claim 7 wherein the metal article is a phosphated metal article.

11'. A metal article which has been corrosion-proofed in accordance with the method claim 7.

12. A metal article is accordance with claim 11 which has been provided with a siccative top-coat.

References Cited by the Examiner UNITED STATES PATENTS 2,773,623 12/1956 Schuster et al. 148-6.2 X 2,868,679 1/-1 959 Pimbley 148-62 3,053,702 9/1962. Schuster et al. 148-62.

ALFRED L. LEAVITT, Primary Examiner.

R. S. KENDALL, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No, 3,282,744 November 1, 1966 Fred Corwin Goldsmith It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 7, line 3, for "comopsition" read composition column 8, line 28, for "DJ" read 0n03 a Signed and sealed this 5th day of September 1967u Attest:

ERNEST W. SWIDER EDWARD J. BRENNER Attesting Officer Commissioner of Patents 

1. A COMPOSITION ADAPTED FOR THE CORROSION-PROOFING OF METAL ARTICLES WHICH CONSISTS ESSENTIALLY OF A DILUTE AQUEOUS SOLUTION OF (A) PARTIALLY REDUCED CHROMIC ACID IN WHICH THE RATIO OF HEXAVALENT CHROMIUM TO TRIVALENT CHROMIUM IS WITHIN THE RANGE FROM 0.5 TO ABOUT 5, AND (B) CALCIUM DICHROMATE; IN WHICH SOLUTION FROM ABOUT 20 TO ABOUT 80 PERCENT OF THE CHROMIUM CONTENT THEREOF IS CONTRIBUTED BY COMPONENT A. 